1. Field of the Invention
This invention is related in general to apparatus for measuring angles. In particular, the invention consists of a novel autocollimator system for minimizing the effects of light scattering while measuring the torsional characteristics of magnetic-head suspension assemblies.
2. Description of the Related Art
The magnetic head slider of a magnetic disk system operates by floating in very close proximity over the surface of the magnetic disk, thereby accurately reading and writing data thereon. While the magnetic head slider is floating disposed substantially in parallel over the disk during operation, it must be able to adjust its attitude to conform to magnetic-disk surface imperfections and dynamic displacements, such as surface vibrations generated by the rotating movement. Therefore, the torsional characteristics of the suspension supporting the slider are critical to the proper functioning of the apparatus and must be maintained within prescribed design specifications to prevent contact with the disk surface and avoid disabling consequences that may result from any such event.
For illustration, FIG. 1 shows in perspective view a conventional magnetic head gimbal assembly 2 (HGA) positioned over a magnetic disk 4. The head gimbal assembly 2 consists of a slider 6 mounted on a gimbal 8 which is either integral with or rigidly connected to a load beam 10 that comprises a pre-load region 12 and formed rails 22 that provide rigidity to the assembly. The combined gimbal and load beam, which constitute the suspension 11, support the slider portion of the head gimbal assembly. The suspension is in turn attached to a driving mechanism (not shown) by means of a screw or swage mount 14. In operation, the head gimbal assembly 2 is moved by the driving mechanism along the radius of the magnetic disk 4 (arrows A1) so that the slider 6 may be placed rapidly over the appropriate read/write tracks in circumferential direction with respect thereto as the disk is rotated in the direction of arrow A2.
For ease of description, the radial, tangential and vertical directions with respect to the surface of disk 4 are referenced in the figures by x, y and z coordinates, respectively. Thus, the magnetic head slider 6 is supported by the gimbal 8 for controlling pitching and rolling movements as the slider's position changes in the radial (x axis) and circumferential (y axis) directions of the magnetic disk 4. When the magnetic disk is rotated, an air spring is created by the air flowing between the surface of the disk and the rails 16 in the magnetic head slider 6, and the torsional characteristics (roll) of the suspension 11 and gimbal 8 must be such that the slider maintains its dynamic attitude through surface imperfections and vibrations of the rotating disk.
Each suspension consists of a metal portion that is formed from a very thin (in the order of 0.05 mm) metal sheet of homogeneous physical structure, thereby producing suspensions and load beams expected to have uniform torsional characteristics. The suspensions are strategically punched or etched to produce desired dynamic responses to forces that cause flexure, and the rigid structural rails 22 are typically formed in the suspension to provide support according to predetermined design criteria. The pre-load region 12 and the gimbal 8 in the suspension are normally bent with respect to the plane of the swage mount 14 to provide a built-in angle toward the disk surface before engagement with the disk 4 (a 13-degree angle is typical). When in use, the suspension is normally displaced to a condition approaching zero-degree deflection. This deflection creates a force against the slider 6 of the assembly that keeps the slider at the desired nominal flying height during operation (see FIG. 1).
As magnetic recording technologies continue to evolve, progressive miniaturization of head gimbal assembly components creates critical challenges. One is the tolerance control on the static attitude parameters of the suspension 11 and gimbal 8 as the slider size is reduced. As the slider 6 becomes smaller, the narrower width between its rails results in smaller differential pressure profiles that produce head gimbal assemblies having flying roll characteristics closely correlated to their static roll attributes. Accordingly, flying attitude characteristics may be predicted well by testing the static attitude of the suspensions under controlled conditions.
Thus, in order to ensure the desired dynamic performance of the suspension (pitch, roll and resonance characteristics), each component of the assembly is manufactured according to specific design specifications and is bench tested for predetermined static parameters. The static attitude of each suspension is measured and compared to allowable tolerances. U.S. Pat. No. 5,636,013 describes an instrument for making such static roll and pitch measurements of a suspension that has been mounted on a supporting base to simulate its flying attitude while operating on a magnetic disk. As illustrated in FIG. 2 in a schematic drawing of such an instrument, the suspension 11 is firmly coupled to the support base 24 through the swage mount 14 and is clamped in a static attitude corresponding to the expected dynamic position in operation. The pitch and roll of a measurement point 26 on the suspension 11 (typically on the gimbal 8) with respect to the support base 24 are then measured by means of a collimated light source 28 and a point-range light source 30 having coincidentally directed beams 32,34. Each light beam is reflected from the measurement point 26 to a corresponding sensor array (such as array 36, receiving the light 38 from the point-range source 30), so that z-height measurements and angle measurements can be obtained by triangulation to determine the roll and pitch characteristics of the gimbal 8. In essence, as clearly understood by those skilled in the art, each measurement consists of determining the exact z position of the measurement point 26 on the surface of the gimbal 8 and the angle of the surface with respect to the support base 24 on which the suspension 11 is mounted. A computer 40 electrically coupled to the autocollimation system and point-range sensor system can be used to perform the necessary calculations in known manner.
FIG. 3 illustrates in schematic form an embodiment of the optical arrangement typically used to implement the autocollimator/point-range sensor system described above. A beam 50 is produced by a single light source 52, typically a solid state laser, and then split by a beamsplitter 54 into a first beam 56 used to carry out the point-range sensor operation and a second beam 58 used to carry out the autocollimation procedure. The beam 56 is reflected by a mirror 60 and directed toward a measurement point 26 on the surface 62 of a test sample 64 (such as the gimbal 8 shown in FIG. 2). The reflection of the beam 56 from the test surface is detected by a sensor array 66 and used by triangulation to determine the z position of the measurement point with respect to a reference plane. The second light beam 58 emerging from the splitter 54 is folded by another splitter 68 and directed substantially orthogonally to the measurement point 26 on the surface 62 of the test sample, so that its reflection, transmitted through the splitter 68 is spotted on a detector 70 and shown on a display screen 72 for measuring the angle of the surface 62 with respect to a known reference surface, as is well understood and practiced in the art. An attenuator 100 is typically used to control the intensity of the light reaching the detector 70.
While the set-up of the prior art is theoretically sound and has proven to be effective and efficient, under certain circumstances it has exhibited problems that diminish its usefulness. In particular, the light from beam 56 may be scattered by surface roughness and interfere with the light from beam 58 reflected at the point of measurement 26, such that the spot produced by the autocollimator light 58 on the sensor 70 may be sufficiently obscured by the stray light from beam 56 to hinder or prevent its identification, either automatically or by an operator observing the display 72. Such a problem is illustrated in FIG. 4, where the image on the display 72 during a measurement shows stray light 74 clouding the image 76 of the autocollimator light reflected from the measurement point 26 on the test surface 62. Another common problem with the embodiment illustrated in FIG. 3 is the effect of stray light on the performance of the light source 52 in the instrument. To the extent that stray rays are be reflected back toward the source 52 by the mirror 68 and the beam splitter 54, they return to the light source and are bounced forward again by its reflective surfaces, thereby producing more scattered light and interfering with the proper illumination of the measurement point. An example of the effect of this phenomenon on the image collected on the display screen 72 is illustrated in FIG. 5.
Therefore, there is still a need for improving the optical characteristics of the autocollimator/point-range sensor system described above. This invention is directed at greatly reducing any interference with the autocollimator beam by light scattered by the point-range sensor beam at the point of measurement on the test surface.